If the bore was nice and clean when you put the plug back in, I am guessing it is either anti seize or something that was in the breech area that got pushed forward.
Take a patch, apply some isopropyl alcohol to it. Now swab the bore real good. Now look down there. If it is clean, apply some gun oil to a clean patch and swab the barrel with that. You should be all set.
WD 40 is a good product. It will displace water in the bore.. hence the WD (water displacer). I personally don't like to use it on triggers or moving parts as extended use over time can develop an almost like sticky varnish finish to it. But in a barrel it should be fine.
Last, I never used a bore snake on any firearm.